Do you know how fucking messy the old Forums are now? Finding anything there is like a needle in a haystack. The guys probably right. There are dozens of uncredited voices in League for the very reason he stated. They do not ask for credit, because they probably get hired on through an agency that gets credit/compesantion as well.
The "famous" VO actors you see listed on the games voice list are there because of that, they're famous. The woman who plays Akali for example, has been in hundreds of other games. She's a famous voice actress (known for roles in WoW, Skyrim, to me anyways) that gets gigs frequently. As I said, my best guess is that the others are either A) Unwilling to credit themselves for a personal reason(which is really the end of the story if thats the case) or B) The third party agency that they work for does not allow them to credit themselves, due to the fact that the the agency that gets them gigs takes a portion of the payment for the VO's. I'm to assume somewhere in their contract there is something stating they cannot credit themselves unless otherwise allowed to do so.
Again, my opinion. It could be option A), which again leaves the discussion exactly where it started. A lot of questions with zero chance of answers.